Differential effects of hypoxia upon contractions evoked by potassium and noradrenaline in rabbit arteries in vitro

Hif1α-dependent mitochondrial acute O2 sensing and signaling to myocyte Ca2+ channels mediate arterial hypoxic vasodilation

Vasodilation in response to low oxygen (O2) tension (hypoxic vasodilation) is an essential homeostatic response of systemic arteries that facilitates O2 supply to tissues according to demand. However, how blood vessels react to O2 deficiency is not well understood. A common belief is that arterial myocytes are O2-sensitive. Supporting this concept, it has been shown that the activity of myocyte L-type Ca2+channels, the main ion channels responsible for vascular contractility, is reversibly inhibited by hypoxia, although the underlying molecular mechanisms have remained elusive. Here, we show that genetic or pharmacological disruption of mitochondrial electron transport selectively abolishes O2 modulation of Ca2+ channels and hypoxic vasodilation. Mitochondria function as O2 sensors and effectors that signal myocyte Ca2+ channels due to constitutive Hif1α-mediated expression of specific electron transport subunit isoforms. These findings reveal the acute O2-sensing mechanisms of vascular cells and may guide new developments in vascular pharmacology.

Acute oxygen sensing by vascular smooth muscle cells

An adequate supply of oxygen (O₂) is essential for most life forms on earth, making the delivery of appropriate levels of O₂ to tissues a fundamental physiological challenge. When O₂ levels in the alveoli and/or blood are low, compensatory adaptive reflexes are produced that increase the uptake of O₂ and its distribution to tissues within a few seconds. This paper analyzes the most important acute vasomotor responses to lack of O₂ (hypoxia): hypoxic pulmonary vasoconstriction (HPV) and hypoxic vasodilation (HVD). HPV affects distal pulmonary (resistance) arteries, with its homeostatic role being to divert blood to well ventilated alveoli to thereby optimize the ventilation/perfusion ratio. HVD is produced in most systemic arteries, in particular in the skeletal muscle, coronary, and cerebral circulations, to increase blood supply to poorly oxygenated tissues. Although vasomotor responses to hypoxia are modulated by endothelial factors and autonomic innervation, it is well established that arterial smooth muscle cells contain an acute O₂ sensing system capable of detecting changes in O₂ tension and to signal membrane ion channels, which in turn regulate cytosolic Ca²⁺ levels and myocyte contraction. Here, we summarize current knowledge on the nature of O₂ sensing and signaling systems underlying acute vasomotor responses to hypoxia. We also discuss similarities and differences existing in O₂ sensors and effectors in the various arterial territories.

Sex-differences in the sympathetic neurocirculatory responses to chemoreflex activation

Abstract

The purpose of this study was to determine whether there are sex differences in the cardiorespiratory and sympathetic neurocirculatory responses to central, peripheral, and combined central and peripheral chemoreflex activation. Ten women (29 ± 6 years, 22.8 ± 2.4 kg/m²: mean ± SD) and 10 men (30 ± 7 years, 24.8 ± 3.2 kg/m²) undertook randomized 5 min breathing trials of: room air (eucapnia), isocapnic hypoxia (10% oxygen (O₂); peripheral chemoreflex activation), hypercapnic hyperoxia (7% carbon dioxide (CO₂), 50% O₂; central chemoreflex activation) and hypercapnic hypoxia (7% CO₂, 10% O₂; central and peripheral chemoreflex activation). Control trials of isocapnic hyperoxia (peripheral chemoreflex inhibition) and hypocapnic hyperoxia (central and peripheral chemoreflex inhibition) were also included. Muscle sympathetic nerve activity (MSNA; microneurography), mean arterial pressure (MAP; finger photoplethysmography) and minute ventilation (V˙_E; pneumotachometer) were measured. Total MSNA (P = 1.000 and P = 0.616), MAP (P = 0.265) and V˙_E (P = 0.587 and P = 0.472) were not different in men and women during eucapnia and during isocapnic hypoxia. Women exhibited attenuated increases in V˙_E during hypercapnic hyperoxia (27.3 ± 6.3 vs. 39.5 ± 7.5 l/min, P < 0.0001) and hypercapnic hypoxia (40.9 ± 9.1 vs. 53.8 ± 13.3 l/min, P < 0.0001) compared with men. However, total MSNA responses were augmented in women (hypercapnic hyperoxia 378 ± 215 vs. 258 ± 107%, P = 0.017; hypercapnic hypoxia 607 ± 290 vs. 362 ± 268%, P < 0.0001). No sex differences in total MSNA, MAP or V˙_E were observed during isocapnic hyperoxia and hypocapnic hyperoxia. Our results indicate that young women have augmented sympathetic responses to central chemoreflex activation, which explains the augmented MSNA response to combined central and peripheral chemoreflex activation.

Key points

Sex differences in the control of breathing have been well studied, but whether there are differences in the sympathetic neurocirculatory responses to chemoreflex activation between healthy women and men is incompletely understood.

We observed that, compared with young men, young women displayed augmented increases in muscle sympathetic nerve activity during both hypercapnic hyperoxia (central chemoreflex activation) and hypercapnic hypoxia (central and peripheral chemoreflex activation) but had attenuated increases in minute ventilation.

In contrast, no sex differences were found in either muscle sympathetic nerve activity or minute ventilation responses to isocapnic hypoxia (peripheral chemoreceptor stimulation).

Young women have blunted ventilator, but augmented sympathetic responses, to central (hypercapnic hyperoxia) and combined central and peripheral chemoreflex activation (hypercapnic hypoxia), compared with young men. The possible causative association between the reduced ventilation and heightened sympathetic responses in young women awaits validation.

Effects of hypoxia, anoxia, and metabolic inhibitors on KATP channels in rat femoral artery myocytes

Vascular ATP-sensitive potassium (KATP) channels have an important role in hypoxic vasodilation. Because KATP channel activity depends on intracellular nucleotide concentration, one hypothesis is that hypoxia activates channels by reducing cellular ATP production. However, this has not been rigorously tested. In this study we measured KATP current in response to hypoxia and modulators of cellular metabolism in single smooth muscle cells from the rat femoral artery by using the whole cell patch-clamp technique. KATP current was not activated by exposure of cells to hypoxic solutions (Po2 approximately 35 mmHg). In contrast, voltage-dependent calcium current and the depolarization-induced rise in intracellular calcium concentration ([Ca2+]i) was inhibited by hypoxia. Blocking mitochondrial ATP production by using the ATP synthase inhibitor oligomycin B (3 microM) did not activate current. Blocking glycolytic ATP production by using 2-deoxy-D-glucose (5 mM) also did not activate current. The protonophore carbonyl cyanide m-chlorophenylhydrazone (1 microM) depolarized the mitochondrial membrane potential and activated KATP current. This activation was reversed by oligomycin B, suggesting it occurred as a consequence of mitochondrial ATP consumption by ATP synthase working in reverse mode. Finally, anoxia induced by dithionite (0.5 mM) also depolarized the mitochondrial membrane potential and activated KATP current. Our data show that: 1) anoxia but not hypoxia activates KATP current in femoral artery myocytes; and 2) inhibition of cellular energy production is insufficient to activate KATP current and that energy consumption is required for current activation. These results suggest that vascular KATP channels are not activated during hypoxia via changes in cell metabolism. Furthermore, part of the relaxant effect of hypoxia on rat femoral artery may be mediated by changes in [Ca2+]i through modulation of calcium channel activity.

Effects of long-term, high-altitude hypoxia on tension and intracellular calcium responses in coronary arteries of fetal and adult sheep

OBJECTIVES

We have previously shown that after exposure to long-term hypoxia, fetal coronary flow is maintained at control levels despite a 25% reduction in cardiac output. We also demonstrated that coronary vascular rings isolated from the long-term hypoxic fetuses and studied in well-oxygenated bath system displayed significantly reduced depolarization-induced contraction strength in response to KCl. To study the mechanism of reduced fetal coronary vascular responses to KCl-induced contractions following exposure to long-term hypoxia, we measured tension and intracellular calcium simultaneously, as well as L-type Ca2+ channel density and sensitivity.

METHODS

Pregnant ewes were housed at altitude (3820 m) for approximately 110 days. At 138 to 141 days of gestation, long-term hypoxic and control animals were killed and fetal and adult left anterior descending coronary artery (LAD) was isolated and studied in a well-oxygenated bath system. Tension and intracellular calcium ([Ca2+]i) were measured simultaneously in response to increasing concentrations of KCl and, in addition, the sensitivity to the calcium channel blocker nifedipine was measured at a half maximal concentration of KCl. We also measured L-type Ca2+ channel density with (+)-[3H]PN200-110.

RESULTS

L-type Ca2+ channel density was decreased by approximately 31% in the long-term hypoxic fetal, but not adult, LAD. Tension in the long-term hypoxic fetal and adult LAD was significantly lower at all concentrations of KCl. [Ca2+]i was lower at rest in both fetal and adult LAD from long-term hypoxic animals and increased to lower levels at all concentrations of KCl. The ratio of tension to [Ca2+]i was also lower at all concentrations of KCl. Sensitivity to nifedipine was unchanged.

CONCLUSIONS

The reduced L-type Ca2+ channel density and the reduced [Ca2+]i response to KCl, as well as the reduced tension response to [Ca2+]i, could potentially be involved in the reduction in depolarization-induced contractions in LAD from long-term hypoxic fetuses. In hypoxic adults, reduced [Ca2+]i and reduced tension response to [Ca2+]i may be involved in the lower tension response to KCl-induced contractions.

H2O2 regulates recombinant Ca2+ channel alpha1C subunits but does not mediate their sensitivity to acute hypoxia

Acute hypoxic inhibition of the pore-forming alpha(1C) subunit of the L-type Ca(2+) channel mediates hypoxic arterial vasodilatation, a physiological response which matches tissue O(2) demand and supply in the systemic vasculature. In numerous O(2)-sensing cell types, reactive O(2) species (ROS) have been proposed as mediators linking lowered O(2) levels with the appropriate cellular response. In this study, we examined the roles of H(2)O(2) and NADPH oxidase as mediators of hypoxic inhibition of recombinant alpha(1C) subunits. Human cardiac L-type Ca(2+) channel alpha(1C) subunits were stably expressed in HEK 293 cells. Ca(2+) currents were recorded using the whole-cell configuration of the patch-clamp technique. Bath application of 100microM H(2)O(2) significantly enhanced depolarisation-evoked Ca(2+) currents in a voltage-dependent manner, while dialysis with 1000Uml(-1) catalase reduced these currents. In the presence of catalase, hypoxic inhibition of Ca(2+) currents was not significantly different compared to non-dialysed controls. The NADPH oxidase inhibitors diphenylene iodonium (10microM) and phenylarsine oxide (5microM) were without effect on either basal Ca(2+) currents or responses to hypoxia. Thus, endogenous production of H(2)O(2) regulates the alpha(1C) subunit. However, neither suppression of H(2)O(2) levels nor inhibition of NADPH oxidase is involved in O(2)-dependent regulation of the Ca(2+) channel.

Failure of systemic hypoxia to blunt alpha-adrenergic vasoconstriction in the human forearm

Systemic hypoxia in humans evokes forearm vasodilatation despite significant reflex increases in sympathetic vasoconstrictor nerve activity and noradrenaline spillover. We sought to determine whether post-junctional alpha-adrenergic vasoconstrictor responsiveness to endogenous noradrenaline release is blunted during systemic hypoxia. To do so, we conducted a two-part study in healthy young adults. In protocol 1, we measured forearm blood flow (FBF; venous occlusion plethysmography) and calculated the vascular conductance (FVC) responses to brachial artery infusions of two doses of tyramine (evokes endogenous noradrenaline release) in 10 adults during normoxia and mild systemic hypoxia (85 % O2 saturation; pulse oximetry of the earlobe). Systemic hypoxia evoked significant forearm vasodilatation as indicated by the increases in FBF and FVC (approximately 20-23 %; P < 0.05). The low and high doses of tyramine evoked significant reductions in FVC (vasoconstriction) that were similar in magnitude during normoxia (-29 +/- 3 and -53 +/- 4 %) and mild hypoxia (-35 +/- 4 and -58 +/- 3 %; P = 0.33). In protocol 2, forearm vasoconstrictor responses to the high dose of tyramine were determined in eight young adults during normoxia and during graded levels of systemic hypoxia (85, 80 and 75 % O2 saturation). The reductions in FVC were similar during normoxia (-59 +/- 2 %) and the three levels of hypoxia (85 % O2 saturation, -64 +/- 3 %; 80 % O2 saturation, -62 +/- 1 %; 75 % O2 saturation, -61 +/- 3 %; P = 0.37). In both protocols, the tyramine-induced increases in deep venous noradrenaline concentrations were similar during normoxia and all levels of hypoxia. Our results demonstrate that post-junctional alpha-adrenergic receptor vasoconstrictor responsiveness to endogenous noradrenaline release is not blunted during mild-to-moderate systemic hypoxia in healthy humans.

Interaction of myogenic mechanisms and hypoxic dilation in rat middle cerebral arteries

The goal of this study was to determine how myogenic responses and vascular responses to reduced Po(2) interact to determine vascular smooth muscle (VSM) transmembrane potential and active tone in isolated middle cerebral arteries from Sprague-Dawley rats. Stepwise elevation of transmural pressure led to depolarization of the VSM cells and myogenic constriction, and reduction of the O(2) concentration of the perfusion and superfusion reservoirs from 21% O(2) to 0% O(2) caused vasodilation and VSM hyperpolarization. Myogenic constriction and VSM depolarization in response to transmural pressure elevation still occurred at reduced Po(2). Arterial dilation in response to reduced Po(2) was not impaired by pressure elevation but was significantly reduced at the lowest transmural pressure (60 mmHg). However, the magnitude of VSM hyperpolarization was unaffected by transmural pressure elevation. This study demonstrates that myogenic activation in response to transmural pressure elevation does not override hypoxic relaxation of middle cerebral arteries and that myogenic responses and hypoxic relaxation can independently regulate vessel diameter despite substantial changes in the other variable.

Cellular mechanism of oxygen sensing

O2 sensing is a fundamental biological process necessary for adaptation of living organisms to variable habitats and physiological situations. Cellular responses to hypoxia can be acute or chronic. Acute responses rely mainly on O2-regulated ion channels, which mediate adaptive changes in cell excitability, contractility, and secretory activity. Chronic responses depend on the modulation of hypoxia-inducible transcription factors, which determine the expression of numerous genes encoding enzymes, transporters and growth factors. O2-regulated ion channels and transcription factors are part of a widely operating signaling system that helps provide sufficient O2 to the tissues and protect the cells against damage due to O2 deficiency. Despite recent advances in the molecular characterization of O2-regulated ion channels and hypoxia-inducible factors, several unanswered questions remain regarding the nature of the O2 sensor molecules and the mechanisms of interaction between the sensors and the effectors. Current models of O2 sensing are based on either a heme protein capable of reversibly binding O2 or the production of oxygen reactive species by NAD(P)H oxidases and mitochondria. Complete molecular characterization of the hypoxia signaling pathways will help elucidate the differential sensitivity to hypoxia of the various cell types and the gradation of the cellular responses to variable levels of PO2. A deeper understanding of the cellular mechanisms of O2 sensing will facilitate the development of new pharmacological tools effective in the treatment of diseases such as stroke or myocardial ischemia caused by localized deficits of O2.

Cytochrome P-450 omega-hydroxylase: a potential O(2) sensor in rat arterioles and skeletal muscle cells

The purposes of this study were to 1) further evaluate the possible role that vasoconstrictor metabolites of cytochrome P-450 (CYP) omega-hydroxylase plays in O(2)-induced constriction of arterioles in the rat skeletal muscle microcirculation, 2) determine whether omega-hydroxylases are expressed in rat cremaster muscle, and 3) determine whether the enzyme is located in the parenchyma or the arterioles. O(2)-induced constriction of third-order arterioles in the in situ cremaster muscle of Sprague-Dawley rats was significantly inhibited by the CYP inhibitors N-methyl-sulfonyl-12,12-dibromododec-11-enamide (DDMS; 50 microM) and 17-octadecynoic acid (ODYA; 10 microM). Immunoblot analysis with antibody raised against CYP4A protein indicated the presence of immunoreactive proteins in the cremaster muscle and in isolated arterioles and muscle fibers from this tissue. However, the molecular mass of the immunoreactive proteins was 85 kDa instead of the expected 50--52 kDa for CYP4A omega-hydroxylase isolated from rat liver or kidney. Treatment of the cremaster muscle with deglycosidases shifted the bands to the expected range which indicates that these proteins are likely glycosylated in skeletal muscle. Immunohistochemistry revealed intense staining of both muscle fibers and microvessels in the cremaster muscle. The results of this study indicate that O(2) sensing in the skeletal muscle microcirculation may be mediated by CYP4A omega-hydroxylases in both arterioles and parenchymal cells.

Cytochrome P-450 omega-hydroxylase senses O2 in hamster muscle, but not cheek pouch epithelium, microcirculation

The goal of this study was to investigate the role of cytochrome P-450 omega-hydroxylase in mediating O2-induced constriction of arterioles in the microcirculation of the hamster. Male Golden hamsters were anesthetized with pentobarbital sodium, and the cremaster muscle or cheek pouch was prepared for observation by intravital microscopy. Arteriolar diameters were measured during elevations of superfusate PO2 from approximately 5 to 150 mmHg. Arteriolar responses to elevated PO2 were determined in the cremaster muscle, in the retractor muscle where it inserts on the cheek pouch, and in the epithelial portion of the cheek pouch. Elevation of superfusion solution PO2 caused a vigorous constriction of arterioles in the cremaster and retractor muscles and in the epithelial portion of the cheek pouch. Superfusion with 10 microM 17-octadecynoic acid, a suicide substrate inhibitor of cytochrome P-450 omega-hydroxylase, and intravenous infusion of N-methylsulfonyl-12,12-dibromododec-11-enamide, a mechanistically different and highly selective inhibitor of cytochrome P-450 omega-hydroxylase, caused a significant reduction in the magnitude of O2-induced constriction of arterioles in the cremaster and retractor muscles. However, arteriolar constriction in response to elevated PO2 was unaffected by 17-octadecynoic acid or N-methylsulfonyl-12,12-dibromododec-11-enamide in the epithelial portion of the cheek pouch. These data confirm that there are regional differences in the mechanism of action of O2 on the microcirculation and indicate that cytochrome P-450 omega-hydroxylase senses O2 in the microcirculation of hamster skeletal muscle, but not in the cheek pouch epithelium.

Effects of chronic hypoxia on Ca2+ mobilization and Ca2+ sensitivity of myofilaments in uterine arteries

The effect of chronic hypoxia on free intracellular Ca2+ concentration ([Ca2+]i) and Ca2+ sensitivity of myofilaments during agonist stimulation was examined in uterine arteries obtained from normoxic and chronically hypoxic pregnant sheep maintained at high altitude (3,820 m) for approximately 110 days. Smooth muscle [Ca2+]i was measured simultaneously with muscle contraction in the same intact tissue. Whereas both KCl and 5-HT increased [Ca2+]i and tension simultaneously in the uterine artery, 5-HT produced significantly greater contractile tension (in g) than KCl at a given amount of [Ca2+]i as indicated by the ratio of fura 2 fluorescence intensity induced by excitation at 340 nm to that induced at 380 nm (29.8 +/- 6.9 vs. 16.9 +/- 4.0, P < 0.05). Chronic hypoxia did not change KCl-induced contractions, nor did it affect KCl-mediated increases in [Ca2+]i. In contrast, chronic hypoxia significantly inhibited 5-HT-induced contractions and decreased the 5-HT-stimulated increase in [Ca2+]i (pD2 7.46 +/- 0.18-->6.86 +/- 0.11, P < 0.05, where pD2 is -log half-maximal effective concentration) in uterine arteries. In addition, the slope (g tension/nM [Ca2+]i) of the 5-HT-mediated [Ca2+]i-tension relationship was significantly decreased in chronically hypoxic arteries (0.024 +/- 0.002-->0.013 +/- 0.001, P < 0.01). The results suggest that chronic hypoxia suppresses agonist-mediated Ca2+ homeostasis in uterine arteries by inhibiting Ca2+ mobilization and the agonist-enhanced Ca2+ sensitivity of myofilaments.

Contrasting effects of hypoxia on cytosolic Ca2+ spikes in conduit and resistance myocytes of the rabbit pulmonary artery

1. The effects of hypoxia on cytosolic Ca2+ ([Ca2+]i) and spontaneous cytosolic Ca2+ spikes were examined in fura 2-loaded myocytes isolated from conduit and resistance branches of the rabbit pulmonary artery. In all myocyte classes, generation of the Ca2+ spikes was modulated by basal [Ca2+]i which, in turn, was influenced by the influx of Ca2+ through L-type Ca2+ channels of the plasmalemma. 2. Conduit and resistance myocytes responded distinctly to hypoxia. In most conduit myocytes (approximately 82% of total; n = 23) exposure to hypoxia reduced basal [Ca2+]i. This effect was often associated with the abolition of the Ca2+ spikes. Hypoxia gave rise to two main responses in resistance myocytes. In a subset of resistance myocytes (41 % of total; n = 34) hypoxia incremented basal [Ca2+]i but reduced Ca2+ spike amplitude. This response mimicked the effect of membrane depolarization with K+ and was reverted by nifedipine or the removal of extracellular Ca2+. In a second subset of resistance myocytes (59% of total; n = 34) hypoxia decreased basal [Ca2+]i and, in most cases, increased spike amplitude; a response counteracted by depolarization with K+. 3. These results indicate that hypoxia can differentially modulate [Ca2+]i in smooth muscle cells from large and small diameter pulmonary vessels through a dual effect on transmembrane Ca2+ influx. Our observations further demonstrate the longitudinal heterogeneity of myocytes along the pulmonary arterial tree and help to explain the hypoxic vasomotor responses in the pulmonary circulation.

Oxygen-sensitive calcium channels in vascular smooth muscle and their possible role in hypoxic arterial relaxation

We have investigated the modifications of cytosolic [Ca2+] and the activity of Ca2+ channels in freshly dispersed arterial myocytes to test whether lowering O2 tension (PO2) directly influences Ca2+ homeostasis in these cells. Unclamped cells loaded with fura-2 AM exhibit oscillations of cytosolic Ca2+ whose frequency depends on extracellular Ca2+ influx. Switching from a PO2 of 150 to 20 mmHg leads to a reversible attenuation of the Ca2+ oscillations. In voltage-clamped cells, hypoxia reversibly reduces the influx of Ca2+ through voltage-dependent channels, which can account for the inhibition of the Ca2+ oscillations. Low PO2 selectively inhibits L-type Ca2+ channel activity, whereas the current mediated by T-type channels is unaltered by hypoxia. The effect of low PO2 on the L-type channels is markedly voltage dependent, being more apparent with moderate depolarizations. These findings demonstrate the existence of O2-sensitive, voltage-dependent, Ca2+ channels in vascular smooth muscle that may critically contribute to the local regulation of circulation.

A new mounting technique for perfusion of isolated small arteries: the effects of flow and oxygen on diameter

There is at present no suitable technique available for performing pressure-flow studies in isolated small arteries (i.e., less than 500 microns), in which the effects of flow and pressure on artery dimensions can be studied independently. A new mounting technique is presented in which the ends of a vessel segment are cemented to the inner surface of two cannulae, with a tip diameter slightly larger than the outer diameter of the vessel, using two-component human fibrin glue. By means of this technique the pressure drop over the cannulae can be made small. First the effect of the glue on constrictive properties is studied. The glue used has no significant influence on the norepinephrine dose-response relation or on the relaxation in response to 1.0 microM acetylcholine. Small mesenteric arteries of the rabbit with outer passive diameters (at zero pressure) of 315 microns (+/- 22 microns SEM) are studied with this method. The effects of flow (shear stress) and oxygen are investigated (vessels are preconstricted (30%) with norepinephrine (1-2 microM)). The flow range used resulted in shear stresses between 0 and 290 dyn.cm-2, a range including values found in vivo. There is a significant (P less than 0.001) decrease in diameter when flow is increased, and hypoxia (pO2 less than 30 mm Hg) augmented the preconstriction with norepinephrine (P = 0.002). The flow effect and the oxygen influence are independent of each other. These results are similar to our previous findings in the femoral artery of the rabbit (diameter about 1200 microns).

Rate-limiting energy-dependent steps controlling oxidative metabolism-contraction coupling in rabbit aorta

1. We tested the hypotheses that coupling between oxidative metabolism and force in noradrenaline (NOR)-activated rabbit aorta is controlled (a) by an energy-dependent step or steps in receptor-operated coupling mechanisms upstream to myosin light chain (MLC) kinase, or (b) by energy limitation of MLC kinase-mediated phosphorylation of the MLC or actin-activated myosin ATPase. 2. Oxidative energy production was rapidly inhibited by decreasing organ bath PO2 to less than 30 mmHg. There was no difference, comparing KCl- or NOR-induced force, in the rates of decrease of [PCr] (phosphocreatine) or [ATP] following inhibition of oxidative energy production. (In this report we use the term [PCr] and [ATP] to indicate mean tissue values). Initial rates of decrease in [PCr] and [ATP] following inhibition of oxidative energy production were 0.05 mM/min and 0.06 mM/min, respectively. 3. Despite similar decreases in mean tissue [PCr] and [ATP], relaxations of KCl-induced contractions following inhibition of oxidative energy production were markedly delayed and were blunted compared to relaxations seen during NOR-induced contractions. The threshold mean tissue [PCr] and [ATP] for relaxation during KCl stimulation were 0.25 and 0.60 to 0.80 mM, respectively. During NOR stimulation, threshold values of [PCr] and [ATP] were 0.50 mM and 0.80 mM, respectively. Mean tissue [PCr] and [ATP] levels at 50% relaxation of KCl-induced force were less than 0.1 mM and 0.1 mM, respectively. Fifty per cent relaxation of NOR-induced force occurred at [PCr] and [ATP] values of 0.35 mM and 0.65 mM, respectively. 4. MLC phosphorylation levels decreased during relaxation of NOR force evoked by inhibition of oxidative energy production. There was no change in the level of MLC phosphorylation following inhibition of oxidative energy production in KCl-contracted muscle even at mean tissue [PCr] and [ATP] lower than values associated with decreases in MLC phosphorylation during relaxations of NOR-induced force. 5. Oxygen-induced redevelopment of force during NOR exposure was not dependent on extracellular Ca2+. Mean tissue [PCr] increased prior to onset of O2-evoked force redevelopment. Increases in MLC phosphorylation were seen at the time of onset of force redevelopment. 6. Oxidative metabolism-contraction coupling during NOR-stimulation seems not to be due to energy limitation of the MLC kinase reaction or actin-activated myosin ATPase. Data suggest the rate-limiting step is an energy-dependent reaction in receptor-operated coupling mechanisms upstream to MLC kinase.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of hypoxia on the contractions and lactate release induced by high-K+, ouabain and epinephrine in guinea-pig aorta

1. The 60 mM K+, 152 mM K+, Na-deficient medium and oubain-induced contractions of aorta were not so affected by severe hypoxia. 2. The 60 mM K+, 152 mM K+, Na(+)-deficient medium-induced responses were greatly reduced by deprivation of external Ca2+ in normoxia. 3. As the concentration of epinephrine increased, the remaining tensions which were expressed as a percentage of the original tensions became progressively greater in hypoxic condition. 4. The percentage of resistant components of the norepinephrine-induced contraction by the lower concentration was further reduced in Ca(2+)-free medium by severe hypoxic condition. 5. The tensions under normoxia and lactate release under severe hypoxia induced by 60 mM K+ or 2.5 x 10(-6) M epinephrine were of the same extent. 6. In conclusion, the inhibition of aortic response to epinephrine with severe hypoxia could not solely be explained by depression of the oxygen supply into the oxidative metabolism. Severe hypoxia did not affect Ca2+ influx through voltage-operated Ca2+ channels, but reduced both receptor-operated Ca2+ influx and intracellular Ca2+ release in the aorta.

Mechanical and biochemical events during hypoxia-induced relaxations of rabbit aorta

Hypoxic relaxation of norepinephrine contractions of isolated rabbit aorta is rapid, whereas relaxation of KCl contractions is slower and blunted. The data given here suggest that with receptor-evoked contractions of rabbit aorta, the energy-limitation of ATP-dependent K+ channels and other sarcolemmal channels, myosin light chain kinase, and actin-activated myosin ATPase are probably not involved in oxidative energy-contraction coupling. The data strongly support the hypothesis that the rate limiting, energy-dependent step is upstream to myosin light chain kinase, which is 50% inhibited at an ATP concentration of about 0.5 mM. This energy-dependent step may be in the inositol phospholipid transduction system, as we have previously postulated (Coburn et al., 1988). In contrast the energy-limited reaction during KCl contractions appears to be the actin-activated myosin ATPase which is 50% inhibited at a mean ATP concentration of about 0.1 mM.

Effects of hypoxia upon contractions evoked in isolated rabbit pulmonary artery by potassium and noradrenaline

1. Comparisons have been made between rabbit thoracic aorta and main pulmonary artery of the effects of hypoxia upon contractions evoked by noradrenaline (NA) and KCl (K+). 2. Contractions were evoked in cylindrical sections of pulmonary artery and aorta, mounted for isometric recording of tension, by NA and K+ (at ED80) in normoxia (PO2 110 mmHg) and hypoxia (PO2 23 or 7 mmHg). Contractions were also evoked in Ca2(+)-free conditions with EGTA to prevent influx of extracellular Ca2+. All contractions are expressed as a percentage of normoxic response in the presence of Ca2+. 3. Potassium-evoked contractions of aorta and pulmonary artery were reduced to a similar extent by both levels of hypoxia, to 85 and 92% respectively. As expected K(+)-evoked contractions were virtually abolished by Ca2(+)-free conditions. It is proposed that hypoxia has a small inhibitory effect upon contraction mediated by Ca2+ influx via voltage-operated Ca2+ channels. 4. In the aorta in the presence of Ca2+, hypoxia reduced NA-evoked contractions to 84% at PO2 23 mmHg and 34% at PO2 7 mmHg. In the absence of Ca2+, NA-evoked contractions reached 73% in normoxia, but only 43 and 21% at PO2 23 and 7 mmHg respectively. These results suggest that hypoxia reduces the component of contraction that is mediated by release of intracellular Ca2+ and possibly that mediated by agonist-induced Ca2+ influx. 5. In the pulmonary artery also, NA-evoked responses in the absence of Ca2+ were reduced from 60% in normoxia, to 49 and 38% at PO2 23 and 7 mmHg. But, in the presence of Ca2+, hypoxia potentiated NA-evoked contractions to 113 and 111% at PO2 23 and 7 mmHg respectively. It is proposed that in the pulmonary artery, hypoxia reduces the component of contraction mediated by release of intracellular Ca2+, but facilitates that mediated by extracellular Ca2+. Possible mechanisms are discussed.